1. Field of the Invention
Head scab, also known as Fusarium head blight (FHB), is a devastating disease of wheat and barley that is primarily caused by the fungus Gibberella zeae (anamorph=Fusarium graminearum). This disease can reach epidemic levels and causes extensive damage to wheat and barley in humid and semi-humid wheat growing areas of the world. In recent growing seasons, the disease has caused large scale devastation in the United States, Canada and China. FHB was responsible for almost 500 million bushels of wheat lost in the United States from 1991 until present. Economic loss has been estimated at between 1.3 to 2.6 billion during this time period. In an epidemic in Indiana in 1986, grain samples from 43 of 44 counties had scab [Tuite et al., (1990) Plant Dis. 74:959-962]. Other countries of the world that produce large amounts of wheat in humid and semi-humid regions and would be susceptible to major outbreaks of FHB include India, Russia, France, Germany and the United Kingdom.
The infection of seed by G. zeae reduces seed germination, seedling vigor and plant emergence [Bechtel et al., (1985) Cereal Chem. 62:191-197]. Infection of wheat kernels by G. zeae reduces grain yield and affects grain quality [Clear et al., (1990) Can. J. Plant Sci. 70:1057-1069]. Reductions in grain yield are at least partially attributable to the pathogen producing the vomitoxin deoxynivalenol (DON) [Snijders, (1990) Neth J. Plant Pathol. 96:187-198; Proctor et al., (1995) MPMI 8:593-601] which can inhibit amino acid incorporation and protein production in plant tissues [Casale et al., (1988) Phytopathology 78:1673-1677]. This toxin is also implicated in adversely affecting the growth of mammalian cells [Knasmuller et al., (1997) Mutation Research 391:39-48]. DON is retained in semolina at approximately 50% and F. graminearum has a strong adverse effect on pasta color when Fusarium damaged kernels make up as little as 2% of a lot [Dexter et al., (1997) Cereal Chem. 74:519-525]. Additionally, G. zeae infected kernels can contain the estrogenic toxin zearalenone. Grain contaminated with either of these mycotoxins often is downgraded or can not be sold [Tuite et al., (1990)]. Contaminated grain is frequently unsuitable for human consumption and may be refused as feed [Vesonder et al., (1980) Process Biochem. 16:12-15]. The importance of FHB was recognized by the 105th U.S. Congress when it adopted the "Wheat and Barley Protection Act" that authorized expenditure of 26 million dollars for the study of FHB.
This invention relates to two Bacillus species that are effective antagonists of Fusarium head blight.
2. Description of the Prior Art
Though some success in controlling FHB can be expected by plowing fields to bury crop residues infested with F. graminearum after harvest [Bai et al., (1994) Plant Dis. 78:760-766], minimal tillage practices render this alternative unacceptable. Some progress has been made in finding and analyzing scab resistance in wheat, though all cultivars in current production are susceptible [Bai et al., (1994)]. Foliar fungicides applied at anthesis can be useful in reducing scab [McMullen, (1998) Fungicide technology network of the National FHB initiative--1998 Report. Proceedings of the 1998 Head Scab Forum, Michigan State University, October 26-27, pp.47-49], but few fungicides are registered for use on wheat this late in the growing season [Shaner et al., (1992) Fungic. Nematicide Tests. 47:206-207]. Additionally, costs and concerns in the public and private sectors over pesticide residues in the environment and in food products render this disease control alternative less attractive.
Biological control, though currently not available, would be an environmentally acceptable method for substantially decreasing the level of disease incited by G. zeae. Though biological control agents (BCA's) have become a more acceptable control alternative for plant pathogens and BCA products are being marketed to a greater extent than ever before [Fravel et al., (1996) Biological and Cultural Tests 11:1-7] to date there have been few attempts to develop strategies and microorganisms for biologically controlling FHB [Stockwell et al., (1997) Phytopathology 87(6):S94; Perondi et al, (1996) Fitopatologia Brasiliera 21:243-249]. The life cycle of G. zeae suggests that the pathogen is especially susceptible to control using applied microorganisms at anthesis through the soft dough stage of kernel development, when the majority of wheat head infection by G. zeae is generally considered to occur [Andersen, (1948) Phytopathology 38:595-611; Arthur, (1981) Indiana Agric. Exp. Stn. Bull 36:129-138]; Fernando et al., (1997) Phytopathology 87(6):S30 (Abstr.)].
Luz et al. [5.sup.th International Congress of Plant Pathology, Abstracts of Papers, p. 348 (1988)] reports in vitro screening in excess of 300 bacteria and yeasts isolated from wheat against F. graminearum. Likewise, Perondi et al. [Anais do 2.degree. Simposio de Controle Biologico, Brasilia, DF, p. 128 (Abstr., 1990); Fitopatologia Brasiliera 21:243-249 (1996)] reported testing microbial strains as possible antagonists against F. graminearum. Promising strains selected by the funnel method and tested in greenhouse studies were shown by Luz et al. [Fitopatologia Brasiliera 15(3)246-247 (1990)] to diminish the severity of wheat scab between 7 and 31% when compared to the control.